The present invention is directed to a method and a display device for optically representing information which display device utilizes a light trap with a member containing fluorescent particles and utilizes activating electrodes to couple light from the light trap.
Passive displays, which do not produce any of their own light but only modulate the light of the environment, must be equipped with an additional illumination if they are also intended to display under favorable brightness or illumination conditions. As a rule, this light source consumes current and therefore may adversely effect one of the advantages of a passive display devices which is an extremely low power demand. Therefore, one solution to this problem is to provide a display which can function with as small amount of artificial light as possible.
The problem of providing light for passive displays was solved in a large degree with the development of a so called "fluorescent activated display" commonly referred to a FLAD. In a FLAD, a fluorescent plate is disposed behind a light valve, which can be switched area-wise between a light transmissive and a light blocking state. This plate contains a fluorescent material and has mirrors provided on each of its four narrow edges and is provided with light exit or decoupling windows which may be in the form of notches or indents on a rear surface, which exit windows are aligned with the switchable valve regions of the light valve. The fluorescent plate will collect a large portion of the ambient light, which impinges on it and by means of the fluorescent scattering and subsequent total reflections at the boundary surfaces will conduct this radiation along its interior to the outlet window where it is coupled in a forward direction with increased intensity at the respective light valves. The brightness factor of a fluorescent plate is given in the first approximation by a ratio of the light collecting surface of the plate to the light emitting surface of the plate. Greater details regarding the construction and operation of a FLAD can be obtained from U.S. Pat. No. 4,142,781, which includes the disclosure of German O.S. No. 25 54 226.
The above mentioned type FLAD display devices are provided with good optical qualities, however, it does not have a particularly high efficiency. The fluorescent plate is partially covered by the light valve and therefore has only a relatively small collective surface. In addition, all the switchable valve regions are always provided with fluorescent light and also these regions which are in a light blocking condition will block the impinging light from reaching the fluorescent plate. The valves employed up to the present time operate with polarizers, which will considerably attenuate any light which passes therethrough. Due to these losses, only a moderate efficiency or yield of approximately 30% is obtained.
If, as disclosed in the device of the U.S. Pat. No. 4,113,360, which includes the disclosure of German Offenlegungsschrift No. 26 13 891, the light valve is eliminated and the light capatured in the fluorescent plate is output coupled only at selected locations, the light-up capabilities of the fluorescent plate are better utilized. According to the display devices of this patent, the output coupling windows are created selectively in response to the desired symbols that are to be displayed. This can be accomplished in two ways. In one embodiment, a membrane is moved by means of electrical force into engagement with a rear surface of the fluorescent plate in response to the application of an electrical force and when engaged with the rear surface will cause a light decoupling in the areas of contact. In another embodiment, ferroelectric ceramic plate has its thickness increased by means of an application of a suitable voltage to engage the back of the fluorescent plate to cause decoupling in those areas of contact. Both of these embodiments supply not only a better light intensity representation but also render possible an information storage, however, they are not simple to produce. They require liquid contact films, special spacing techniques, and allow scattering angle distributions within only certain limits. Moreover, the continual flexing of the membrane subjects it to possible damage and in the embodiment using a ferroelectric ceramic materials, relatively high voltages are required.